What is the Joint Investigative Mechanism?

Created in 2015 with unanimous backing from the UN Security Council and renewed in 2016 for another year

Involves the UN and the Organisation for the Prohibition of Chemical Weapons

Has previously concluded that Syrian government forces used chlorine as a weapon at least three times between 2014 and 2015

It has also found that Islamic State militants used sulphur mustard in one attack

Russia, the UK, China, France and the US all have veto powers at the Security Council.

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Media captionAbo Rabeea says he is still suffering from the suspected chemical weapons strike in Khan Sheikhoun

The attack on Khan Sheikhoun left more than 80 people dead and prompted the US to launch missile strikes on a Syrian airbase.

Last month, a UN Human Rights Council inquiry concluded a Syrian air force jet was responsible, dismissing statements from Russia that the jet had dropped conventional munitions that struck a rebel chemical weapons depot.

Syrian President Bashar al-Assad has said the incident in Khan Sheikhoun was a "fabrication".